FIELD OF THE INVENTION
The invention relates to a fossil-fueled steam generator with a vertical gas flue or draft, including an encompassing wall having tubes through which a medium can flow.
The encompassing wall, which typically forms an evaporator heating surface of a steam generator, is often exposed to variably strong heating from one heating element to another. In the lower region near the flame, in which a number of burners for a fossil fuel are disposed, the heating is usually substantially stronger than in the region above it, which is far from the flame. The reason therefor is also that in the region far from the flame, additional exchanger surfaces are often provided, which shield the encompassing wall against overly intensive heating, especially from heat radiation.
The tubes of the encompassing wall through which the medium flows from bottom to top in parallel are welded together on their long sides, to form a gas-tight tube wall typically extending over the entire vertical length of the gas flue. The tube wall is constructed either as a tube-rib-tube structure, with longitudinal plates secured to the tubes, or as a finned tube structure, with fins formed onto the tubes.
One possibility for reducing fuel consumption and therefore also for reducing carbon dioxide emissions from a fossil-fueled steam plant is to increase the steam pressure at an outlet from the steam generator from approximately 250 bar, as before, to 300 bar, for instance. However, the steam temperature also rises in the evaporator region of the steam generator along with the steam pressure, so that it is no longer possible, as was previously usual, to use materials with a low chromium content, which are distinguished by ease of processing. A further difficulty is that because of variously strong heating of the tubes within a cross-sectional plane of the encompassing wall, the evaporation point in the tubes, that is the transition from the liquid to the gaseous state, is located at different points in the flow direction of the medium. As a result, considerable temperature differences can arise between adjacent tubes and therefore there can be differing thermal expansions of the tubes. That in turn can lead to the formation of cracks inside the tube wall.
In order to avoid operation-dictated damage to the tube wall yet nevertheless to attain a fresh steam state that is especially favorable for low coal consumption and low carbon dioxide emissions, for instance with a steam pressure of 300 bar and a steam temperature of 600.degree. C., the use of materials with a high chromium content has already been proposed. However, manufacturing a gas-tight tube wall from such a material must be performed especially carefully and is therefore expensive, especially since an additional heat treatment after welding of the tubes is required. Moreover, long down times are needed for inspection or operation-dictated repairs. That in turn has a deleterious effect on the availability of the steam generator and therefore on the plant availability of the entire steam power plant.